This invention relates to a process for the purification and separation of the diastereomers of 2[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol.
This compound and the salts thereof are of pharmaceutical interest. Tramadol hydrochloride, CA no. 36282-47-0 (xc2x1)-trans-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol hydrochloride has long been commercially available as a highly effective analgesic.
The above designation of the cis/trans isomers is not in accordance with IUPAC nomenclature. Compound CA no. 36282-47-0 is accordingly hereinafter designated the cis isomer or cis-tramadol in accordance with IUPAC nomenclature and taken to encompass the racemic mixture of (1R,2R)- and (1S,2S)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol, while the racemic mixture of (1R,2S)- and (1S,2R)-2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol is designated the trans isomer or trans-tramadol.
Known purification and separation processes, e.g., U.S. Pat. No. 3,830,934 and DE-OS 4 330 240, for the aforementioned tramadol hydrochloride are based on the reaction of the diastereoisomeric base mixture with mineral acids and subsequent fractional crystallization from organic solvents. The disadvantage of this procedure is that it gives rise to two or more fractions which require further processing, so greatly reducing economic viability. Moreover, since concentrated mineral acids are sometimes used, this procedure inevitably gives rise to decomposition products due to the acid lability of the tertiary alcohol function. Furthermore, separation of the cis/trans isomers may only be achieved by means of the above-stated process if the isomer ratio of the base mixture to be separated is greater than 75:25 cis:trans.
WO 99/03820 describes a process for the production of pure cis-tramadol hydrochloride, in which a monohydrate of the corresponding cis-tramadol base is obtained from the Grignard bases by addition of water and is separated.
In all previously known processes, the trans:trans isomer ratio of the diastereomeric base mixtures to be separated is 80:20 and above. For example, according to DE-OS 4 330 240 or U.S. Pat. No. 5,414,129, the ratio is 86:14.
However, there is also interest in separating the desired cis isomer from mixtures which contain the diastereomeric bases in an unfavorable isomer ratio, in order, for example, to be able to work up Grignard reaction mixtures obtained from deviant reaction conditions or also mother liquors. Mother liquors arising from a first precipitation of the desired isomer still contain the diastereomeric bases in a cis:trans isomer ratio of approx. 50:50.
The object of the invention is accordingly to provide a process which, without primary salt formation, permits separation of the diastereomers from a diastereomeric base mixture of 2-[(dimethylamino)methyl]1-(3-methoxyphenyl)cyclohexanol over a wide isomer ratio range.
It has surprisingly been found that diastereomer separation with 2-[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol advantageously proceeds by formation of a base hydrate by adding water or preferably organic solvent or solvent mixture and water to a diastereomeric base mixture of this compound with a cis:trans isomer ratio of below 80:20.
The invention accordingly provides a process for the separation of the diastereomeric bases of 2[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol by treatment with water in at least stoichiometric quantities for complete conversion of the bases and subsequent separation of the precipitated hydrate of the cis diastereomer, which process is characterised in that a base mixture with a cis:trans isomer ratio of below 80:20, preferably of 60:40-75:25, is used.
When separating contaminated diastereomeric base mixtures, said mixtures are preferably dissolved in a water-miscible organic solvent or solvent mixture before the reaction with water, wherein solvents from the group of alcohols, ketones, esters, ethers, low molecular weight polyalcohols or aromatic hydrocarbons are used. The organic solvent or solvent mixture is here preferably used in a volume ratio to water of 10:2-10:5.
The water-miscible organic solvents used are preferably C1-8 alcohols, C3-8 ketones, C2-8 esters, aliphatic, aromatic, open-chain and cyclic C4-8 ethers, C2-6 polyalcohols or C6-9 aromatics.
The separated hydrate crystals of the cis diastereomer are finally washed with a mixture of the organic solvent and water in a volume ratio of 10:2-10:5 and then dried.
The water is used in at least stoichiometric quantities for complete conversion of the bases. Moreover, the water and also the solvent or solvent mixture may be used with the diastereomeric base mixture in a wide range of mixing ratios.
Separation of the diastereomers may proceed over a wide temperature range, provided that it is ensured that the reaction mixture does not freeze out at low temperatures. At higher temperatures, the temperature of the reaction mixture is preferably maintained below the melting point of the base hydrate.
The process according to the invention is characterised in that, under the described conditions, the diastereomeric base mixture forms a hydrate and this hydrate, namely the cis diastereomer, preferably precipitates, so enabling easy separation of the diastereomers.
In comparison with heretofore described procedures, the process offers the advantages that diastereomer separation may proceed without salt formation (for example via the hydrochloride), that unwanted decomposition products are simply and effectively avoided by the formation of the base hydrate and that subsequent salt formation with numerous acids is possible directly via the base stage. Above all, however, the process makes it possible to separate diastereomeric base mixtures, the cis:trans isomer ratio of which deviates greatly from that conventionally arising after the Grignard reaction. In particular, the process is suitable for working up mother liquors.